A stem cell is a generic name for an undifferentiated type of cells found in tissues of embryos, fetuses and adults, which are characterized by the ability to differentiate into a diverse range of specialized cell types. Stem cells may be classified according to various criteria. One of the most frequently used criteria is a source from which stem cells are derived. The two broad types of mammalian stem cells are: embryonic stem cells (ES cells) that are isolated from the inner cell mass of blastocysts, and adult stem cells that are isolated from adult somatic cells. Potency allows the classification of stem cells: pluripotent stem cells, multipotent stem cells, and unipotent stem cells. Pluripotent stem cells can differentiate into cells derived from any of the three germ layers. Embryonic stem cells are representative of pluripotent stem cells. Adult stem cells show multipotency or unipotency.
ES cells are pluripotent stem cells with the potency to differentiate into cells of all tissues constituting the body. However, the preparation of ES cells results in destruction of the fertilized human embryo, which raises ethical issues. In addition, derivation from limited oocytes is an obstacle regarding the use of ES cells in the development of cell therapeutics due to the lack of immune compatibility, which gives rise to transplant rejection. To avoid these problems, a variety of alternatives have been developed for reprogramming adult somatic cells into pluripotent stem cells mimicking embryonic stem cells.
Representative among them are adult somatic cells nuclear transfer (SCNT), fusion with ES cells, and reprogramming by defined factors. SCNT requires a large amount of oocytes due to the very low efficiency thereof. Fusion with ES cells is problematic in terms of stability because the pluripotent cells induced thereby contain two additional pairs of genes. Characterized by the induction of reprogramming with defined genes, the reprogramming by defined factors, which employs a viral delivery system containing a potent oncogene may cause tumorigenicity. Further, low efficiency and methodological difficulty leads to a significant problem in the practical availability of the reprogramming by defined factors in the development of cell therapeutics.